Process for separation of meta-para-xylene isomers from their mixture

ABSTRACT

IN THE SEPARATION OF PARA-XYLENE FROM MIXTURES CONSISTING PREDOMINANTLY OF META- AND PARA-XYLENE ISOMERS BY FRACTIONAL CRYSTALLIZATION, SMALL AMOUNTS OF GASEOUS EUTECTIC INHIBITORS ARE DISSOLVED IN THE XYLENE MIXTURE TO LOWER TO EUTECTIC POINT OF THE MIXTURE AND IMPROVE RECOVERY OF PARA-XYLENE THEREFROM, THE AMOUNT OF GAS DISSOLVED IN THE XYLENE MIXTURE IS BETWEEN ABOUT 0.01 AND ABOUT 4.0 MOLE PRECENT AND IS PREFERABLY IN THE RANGE BETWEEN ABOUT 0.3 AND ABOUT 1.0 MOLE PRECENT.

3,574,778 PROCESS FOR SEPARATION OF META-PARA- XYLENE ISOMERS FROM THEIRMIXTURE Charaniit Rai, Somerset, and Mohamed M. Elmogazi,

Hightstown, N.J., assignors to Cities Service Oil Company, Tulsa, Okla.No Drawing. Filed Nov. 26, 1968, Ser. No. 779,214

Int. Cl. C07c 7/14 US. Cl. 260-674 12 Claims ABSTRACT OF THE DISCLOSUREIn the separation of para-xylene from mixtures consisting predominantlyof metaand para-xylene isomers by fractional crystallization, smallamounts of gaseous eutectic inhibitors are dissolved in the xylenemixture to lower the eutectic point of the mixture and improve recoveryof para-xylene therefrom, The amount of gas dissolved in the xylenemixture is between about 0.01

and about 4.0 mole percent and is preferably in the range between about0.3 and about 1.0 mole percent.

Large quantities of hydrocarbon mixtures consisting predominantly ofxylene isomers are produced commercially. Such mixtures are, forinstance, recovered by fractionation of coke oven distillates andcatalytically reformed naphtha. Recovery of para-xylene from mixtures ofxylene isomers is particularly difficult due to the closely relatedboiling points of the paraand meta isomers. Numerous processes have beenproposed for effecting separation of xylene isomers and particularly,the recovery of para-xylene from such mixtures. Among the mostsuccessful commercially, have been processes based upon fractionalcrystallization effected by cooling the xylene mixture to crystallizethe para-xylene. In the several known processes for fractionalcrystallization of para-xylene, the efficiency of the process isultimately limited by the phase equilibria conditions and especially theeutectic point of the particular xylene mixture from which crystals areobtained.

It has now been found that dissolving small quantities of normallygaseous eutectic inhibitor in mixtures of xylene isomers acts to lowerthe effective eutectic freezing point of the mixtures. This increasesthe yield of paraxylene obtainable from such mixtures by fractionalcrystallization processes and allows such processes to be carried outbelow the normal eutectic point of the mixture of meta-xylene andpara-xylene being treated.

Mixtures of xylene isomers treated for recovery of para-xylene inaccordance with this invention may be obtained from any suitable sourceand contain essentially metaand para-xylene isomers with usually no morethan about vol. percent and preferably no more than about 1 vol. percentof other materials being present. For instance, the invention isapplicable in recovery of paraxylene from mixtures consistingessentially of para-xylene and meta-xylene or from mixtures ofpara-xylene and meta-xylene with small amounts of ethylbenzene and orthoxylene. A typical C aromatics fraction recovered by fractionation orextraction from catalytically reformed naphtha may, for example, containbetween about 10 and about 25 volume percent (vol. percent) ethylbenzene(boling point 277 F.), between about and about vol. percent para-xylene(boiling point 281 F.), between about and about 50 vol. percentmeta-xylene (boiling point 282 F.) and between about 15 and about 25vol. percent ortho-xylene (boiling point 292 F.). Oommercial processesare available for recovery by conventional fractionation of ethylbenzeneand ortho xylene so that these isomers may be removed from the mixturein United States Patent 0 whole or in part before the recovery ofpara-xylene by fractional crystallization. A typical ethylbenzenefractionating column requires 350-400 trays and a very high refluxratio, and an ortho xylene tower has 125-150 trays and a fairly highreflux ratio. The close boiling points of Inetaand para-xylenes make itvirtually impossible to separate these two isomers by distillation. An800 tray fractionating column would be required to completely separatemeta and para-xylene and would be prohibitively expensive. On the otherhand, because of the wide differences in the freezing points between themeta and para isomers (53 F. for meta-xylene and +56 F. for para-xylene)technology has been developed to sepa rate para-xylene bycrystallization. In the recovery of para-xylene from a mixture ofmetaand para-isomers, the temperature of crystallization is usuallylimited to no lower than 2 to 5 F. above the freezing point of theeutectic mixture. At the eutectic freezing point (63 F.), the motherliquor contains 84% meta-xylene and 16% para-xylene.

To be effective in reducing the eutectic point of mixtures of metaandpara-xylene and increasing recovery of para-xylene in accordance withthe invention, the gaseous eutectic inhibitor should be dissolved in thexylene mixture prior to recovery of para-xylene crystals therefrom inamounts between about 0.01 and about 4 mol percent gas based on themixture of xylene isomers, preferably between about 0.3 and 1.0 molepercent. The gas may be dissolved in the mixture by any suitable meanssuch as by addition to the mixture prior to cooling or by directaddition to the crystallization zone. Any normally gaseous materialwhich can be dissolved in the xylene mixture in the quantities mentionedabove at crystallization temperatures and which does not reactchemically with the xylenes under the experimental conditions issuitable for this purpose. Suitable gases include but are not limited toammonia, hydrocarbon gases of suitable solubility such as methane,ethane, ethylene, propane, propylene, methylacetylene, butane,isobutane, butenes, isobutenes, ammonia, carbon dioxide,monomethylamine, butadiene, chlorofiuoromethane and chlorofiuorethenes.Carbonyl sulfide, hydrogen sulfide, hydrogen chloride, hydrogen bromide,dimethyl ether, borontrifluoride, borotrichloride, air, sulfur oxides,vinyl chlorides, methyl bromide, carbon dioxide, oxygen and nitrogenare, for instance, also suitable.

By dissolving gas in a mixture of paraand metaxylenes, the eutecticpoint may be lowered from the normal eutectic point of 63 F. to as lowas about 110 F.

The benefits of this invention are obtained by recovering para-xylenecrystals from mixtures of xylenes at temperatures below the normaleutectic points of such mixtures. Temperatures between about and aboutF. are preferred for this purpose. The amount of para-xylene recoveredfrom a given mixture of xylenes by a given crystallization technique mayfrequently be increased by as much as 100% or more by the use ofdissolved gas in accordance with this invention.

Conventional processes for recovery of para-xylene by fractionalcrystallization may include one or more stages of crystallization,crystal washing and other known techniques for obtaining maximumrecovery of paraxylene crystals of maximum purity. It should beunderstood that all such techniques are equally applicable whenpracticing the present invention and that the benefits of the inventionare obtained in addition to, not in place of, benefits obtained by suchtechniques.

EXAMPLE 1 In order to demonstrate the advantages of the presentinvantion in recovery of para-xylene from mixtures of "ice xyleneisomers and especially the effect of dissolved gases in reducing theeutectic point of xylene mixtures, experiments were carried out directedtoward the separation of additional para-xylene from a mixture ofmeta-xylene and para-xylene containing 16 vol. percent para-xylene and84 vol. percent meta-xylene. This mixture represented the mother liquorfrom conventional recovery of paraxylene by fractional crystallizationat the normal eutectic freezing temperature of this mixture of -63 F.and contained in addition to the metaand para-xylene, about 0.5 vol.percent toluene and ethyl benzene. Attempts were made to recoveradditional para-xylene from this eutectic mixture by lowering thetemperature to about 100 F. with and without addition of dissolved gasaccording to the invention in the following manner.

100 milliliters of the eutectic mixture described immediately above wereplaced in a 200 milliliter tall beaker. The beaker was fitted with athree hole rubberstopper wrapped in aluminum foil. A thermometer, metalwire stirrer and a coarse fritted glass sparger tube were fitted inthese holes. The stirred mixture was cooled in a Dewar flask whichcontained a Dry Ice-acetone mixture with the desired crystallizationtemperature being maintained at about --100 F. for one hour. The spargerwas used in some runs to bubble air, methane, acetylene, propane, etc.,through the xylene mixture for a period of time between about 40 andabout 60 minutes. The flow rate of the gas was twenty milliliters perminute. At the end of each run, the stirring was stopped and thecrystals were separated from the filtrate by connecting a vacuum line tothe sparger tube. Both filtrate and crystals were weighed after warmingto room temperature. The percentage of para-xylene in the crystals andthe filtrate was analyzed by infrared spectrographic techniques and thepercentage of para-xylene recovered was calculated by using theanalytical data on the crystals and the filtrate fractions. The resultsof these runs using air, methane, propane, acetylene and ammonia as wellas the result of the control run in which no gas was dissolved in thexylene mixture are reported in Table I below:

TABLE I.RECOVERY OF PA RA-XYLENE FROM MIXTURE OF PA RAXYLENE ANDMETA-XYLENE Wt. percent Pare-xylene Para-xylene Gas dissolved in xylenerecovered (wt. mixture Crystals Filtrate percent) None Methane 42. 2 7.7 67 48. 7 6. 7 68 Propane 47. 9 7. O 67 Acetylene. 53. 0 6. 4 67Ammonia 50. 0 8. 60

EXAMPLE 2 For this example, a typical mixture of 75 vol. percentmeta-xylene and 25 vol. percent para-xylene is treated forcrystallization of para-xylene therefrom by a conventional fractionalcrystallization process at a temperature of -61 F. Ultimate recovery ofpara-xylene is only about 9 wt. percent based on total feed. Bycontrast, when the same crystallization process is operated with 0.5mole percent propane dissolved in the xylene mixture in accordance withthe present invention, ultimate recovery of para-xylene is about 18 wt.percent based on total feed.

While the invention has been described above with respect to preferredembodiments thereof, it will be appreciated by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention and it is intended to coverall such changes and modifications in the appended claims.

We claim:

1. In a process for the separation of para-xylene from a hydrocarbonmixture consisting predominantly of metaand para-xylene isomers in whichsuch mixture is cooled to thereby crystallize para-xylene andpara-xylene crystals are separated from the mother liquor, theimprovement which comprises dissolving in the mixture between about 0.01and 4 mole percent of a gaseous eutectic inhibitor selected from thegroup consisting of propane, acetylene, methane, propylene, ethane,ammonia, carbon dioxide and air and cooling the mixture to below thenormal eutectic point of such mixture before separation of theparaxylene crystals from the mother liquor.

2. The process of claim 1 in which the hydrocarbon mixture contains atleast about 95 vol. percent metaand para-xylene.

'3. The process of claim 1 in which the normally gaseous eutecticinhibitor is dissolved in the mixture in amounts between about 0.3 andabout 1.0 mol. percent.

4. The process of claim 3 in which the mixture is cooled to betweenabout and about 1l0 F. before separation of para-xylene crystalstherefrom.

5. The process of claim 4 in which the gaseous eutectic inhibitor ispropane.

6. The process of claim 4 in which the gaseous eutectic inhibitor isacetylene.

7. The process of claim 4 in which the gaseous eutectic inhibitor ismethane.

8. The process of claim 4 in which the gaseous eutectic material ispropylene.

9. The process of claim 4 in which the gaseous eutectic inhibitor isethane.

10. The process of claim 4 in which the gaseous eutectic material isammonia.

11. The process of claim 4 in which the gaseous eutectic inhibitor iscarbon dioxide.

12. The process of claim 4 in which the gaseous eutectic inhibitor isair.

References Cited UNITED STATES PATENTS 2,622,115 12/1952 Carney 260-6743,414,630 12/1968 Szawlowski et a1. 260-674 3,467,724 9/1969 Laurich260-674 DELBERT E. GA-NTZ, Primary Examiner C. E. SPRESSER, JR.,Assistant Examiner

